Conventionally, for example, in an electric power steering device, an electric motor is provided for applying a steering assist torque to a steering mechanism, and a steering operation by a driver is assisted by controlling energization of the electric motor. Such an electric power steering device is powered by a vehicle battery for supplying an electric power to general vehicle electric loads, and the consumption of the electric power is large. Accordingly, for example, in a device proposed in the Unexamined Japanese Patent Publication No. 2007-91122, a sub battery device is provided for assisting the vehicle battery. The sub battery device is constituted such that it is connected to a power supply line in parallel thereto from the vehicle battery to a motor drive circuit and is charged by the vehicle battery, and can supply an electric power to the motor drive circuit by using the charged electric energy. Further, in this electric power steering device, a booster circuit is provided for boosting a voltage of an electric power output from the vehicle battery to supply the boosted electric power to the motor drive circuit.